1. Field of the Invention
The present invention relates to a pulse generator for generating pulses with which to control the switching operation of a DC/DC converter designed to step up a direct-current voltage.
1. Description of the Prior Art
A DC/DC converter designed to step up a direct-current (hereafter referred to as "DC") voltage typically has an inductance coil and a switch connected in series between a DC voltage input terminal and ground. The node between the inductance coil and the switch is connected to a rectifier circuit. The rectified output voltage is fed to an output terminal. The switch is typically realized by the use of a switching transistor, and the output voltage varies according to the ratio of the length of the period in which the transistor Tr is conducting (on) and the length of the period in which the transistor Tr is cut off (off). The transistor Tr is turned on or off in synchronism with pulses (duty pulses) that are generated by a pulse generator circuit. The pulse generator circuit generates duty pulses by comparing with a reference voltage a voltage having a sawtooth-shaped waveform generated by a sawtooth-shaped wave generator circuit.
Here, if the duty factor of the period in which the switching transistor Tr is kept on, e.g. the duty factor of the high-level period of the pulses, is higher than a predetermined value, the inductance coil reaches magnetic saturation. Therefore, in a pulse generator circuit, it is not allowed to set the duty factor above a certain limit. Moreover, in cases where the rectified output voltage is fed back so as to be related to the reference voltage fed to the comparator, if the pulse generator circuit is started with the output voltage at 0 (zero) V, unless a limit is imposed on the duty factor of the period in which the switching transistor is kept on, the switching transistor is kept on perpetually by the polarity that is fed back, with the result that no change ever occurs in the output voltage. This means that a pulse generator circuit must be provided with a function of limiting the duty factor (in the example given above, the duty factor of the high-level period of the pulses).
To achieve this, in a conventional pulse generator circuit, it is customary to feed a DC voltage to be used to limit the duty factor (hereafter such a voltage will be referred to as a "duty-factor limiting voltage") to the comparator thereof from the outside so that, when the stepped-up (output) voltage becomes too low, making the reference voltage higher than the duty-factor limiting voltage, the comparator will compare the voltage of the sawtooth-shaped wave with the duty-factor limiting voltage rather than with the reference voltage. This helps prevent the duty factor of the high-level period of the output pulses from becoming higher than a predetermined value.
Thus, a conventional pulse generator circuit as is used to control the output voltage of a DC/DC converter or the like on the basis of feedback requires a separate duty-factor limiting voltage to limit the duty factor of its output pulses. Since the DC level and the amplitude of a sawtooth-shaped wave respectively vary according to how the supply voltage is set and whether synchronization with a clock is required or not, to obtain the desired maximum duty factor, it is necessary to adjust the duty-factor limiting voltage in view of the specifications actually given in particular cases.
This inconveniently leads to a large variation of the maximum duty factor due to temperature variation and improper adjustment. Note that the variation in the maximum duty factor due to temperature variation results from temperature-dependent variation of the duty-factor limiting voltage, which is a DC voltage.
Moreover, a terminal for receiving the duty-factor limiting voltage needs to be provided separately, and the comparator requires a complicate circuit configuration to achieve comparison between the output voltage of an error amplifier and the voltage of the sawtooth-shaped wave. This inconveniently leads to a larger circuit area, and to a higher cost.
Note that synchronization with a clock means synchronizing the period of the switching operation of the DC/DC converter with an even number of times the period of the system clock used in a given appliance. This helps prevent the DC/DC converter from affecting adversely the signals generated within the appliance, suppress temperature-dependent instability of the operation of the DC/DC converter, and achieve other purposes.
Note that synchronization with a clock, when actually achieved, makes the amplitude of the sawtooth-shaped wave smaller. This means that, even if the duty-factor limiting voltage is kept constant, the maximum duty factor varies depending on the presence of synchronization with a clock (i.e. whether synchronization with a clock is being actually achieved or not). For example, in a pulse generator circuit of a certain type, starting synchronization with a clock while keeping the duty-factor limiting voltage at the same level as when the circuit is operating independently (i.e. when synchronization with a clock is not achieved) may make the maximum duty factor of the high level period unduly low.
Moreover, even with the same specifications, if the DC level and the amplitude of the sawtooth-shaped wave vary from appliance to appliance, to obtain the desired maximum duty factor, it is inevitable to adjust the duty-factor limiting voltage for each individual appliance. Therefore, no appliance-to-appliance variation is permitted in the DC level and the amplitude of the sawtooth-shaped wave, and thus it is necessary to use a high-precision voltage generator circuit or the like to generate the sawtooth-shaped wave. This inconveniently requires an even larger circuit area and an even higher cost.
Moreover, since a pulse generator circuit that is used to control the output voltage of a DC/DC converter on the basis of feedback suffers from problems as described above, the DC/DC converter as a whole suffers from instability of its output voltage, from an unduly large circuit area and an unduly high cost, and from the difficulty of the initial adjustments that need to be made when it is incorporated into an appliance.